Same-sex behaviour among primates is neither as unusual nor as mysterious as it was once thought to be, researchers say
Japanese macaques are known for sexual relationships between females, while bonobos use intimacy of all kinds — including males interacting with males — to lubricate the wheels of social life.
This kind of behaviour was once filed away as unusual in the animal kingdom and, from an evolutionary point of view, puzzling. Sex that does not directly lead to reproduction can look, when viewed through a Darwinian lens, like wasted effort.
A study led by researchers at Imperial College London has reached a different conclusion. After analysing data from nearly 500 species of non-human primates, the authors argue that same-sex sexual behaviour is neither particularly rare nor, perhaps, especially mysterious.
Instead, it appears to be a recurring feature of primate life, especially among species facing harsh conditions and living in environments in which strong social relationships may make the difference between coping and failing.
The researchers found reliable evidence of such behaviour in 59 species across the primate family tree, from lemurs to apes, suggesting it is something that has emerged repeatedly over the course of evolution.
Clear patterns emerged. Same-sex sexual behaviour was most likely in primates that lived long lives, showed pronounced physical differences between males and females, and inhabited complex social worlds.
It was also more common where life was hard: in drier environments, where food was less predictable, and where predators posed a persistent threat. In other words, it was most visible in places where cooperation between members of your group might be especially valuable.
One plausible explanation is that same-sex sexual behaviour helps build alliances and strengthen social cohesion, improving a group’s chances when conditions are tough.
Social complexity turned out to be the strongest predictor. Species living in large groups with clear dominance hierarchies were far more likely to show same-sex sexual behaviour than solitary or pair-living species. In such societies, sexual behaviour may serve as a way to reduce aggression, signal alliances or reinforce bonds, the researchers said.
On top of that, in long-lived species, individuals must maintain relationships over many years, often while navigating shifting hierarchies and rivalries. This can be stressful, energy-sapping work. Behaviours that reduce tension or reinforce bonds may pay off over time, even if they do not produce offspring.
The researchers are careful about what their findings show. Published in the journal Nature Ecology & Evolution, the study concerns animal behaviour, not human sexual identity. Still, it inevitably raises broader questions.
Early human ancestors probably faced many of the same pressures as other primates: scarce resources, predators and the stressful challenge of living in hierarchical groups. If those conditions favour same-sex sexual behaviour in primates today, they might also have shaped its emergence in the distant human past — though this, the researchers stress, is entirely speculative.
What is clearer is that same-sex sexual behaviour is not confined to primates. The study also notes that it has been documented in roughly 1,500 animal species overall. Sex, it seems, is not always about reproduction. Quite often, it may be about getting along.
(Source: The Times UK)
